What do static, residual and test flow mean?

They are the three readings from a hydrant flow test. Static is the pressure with no flow, test flow is the discharge measured while flowing, and residual is the pressure recorded at that flow. Those three numbers define the whole curve.

Should I apply a safety margin to the test?

Most designers deduct 5 to 10 psi from the static reading to allow for seasonal variation and future demand, and many authorities having jurisdiction require it. The full app has a safety field so the deduction is recorded on the report rather than done in your head.

Does the hydrant elevation matter?

Yes. If the test hydrant is at a different elevation from the base of the riser, the available pressure must be corrected by that difference before it is compared against the demand. This page plots the raw test; the full app asks for both elevations.

How old can a flow test be?

That is set by your authority having jurisdiction, and one year is a common limit. The full app records the test date and location on the report so the reviewer can judge it.

What is the flow at zero pressure?

Where the supply curve crosses the flow axis: the theoretical maximum the main would deliver if the pressure fell to nothing. It is a reference point for reading the curve, not a flow anyone designs to.

Water supply curve plotter

Plot a hydrant flow test on the N^1.85 scale and read the available pressure at any flow.

Preliminary. The reference tables behind this calculation are pending a second check against their printed source. Verify against your adopted edition before using a result in a submitted calculation.

71.0psi

available at 412.0 gpm

Flow at zero pressure: 2311.2gpm
Pressure drop at the test: 16.0psi
Axis: Q^1.85

Supply 71 psi at 412 gpm, from a flow test reading 74 psi static and 58 psi residual at 1010 gpm.

The formula

P(Q) = Ps - (Ps - Pr) (Q / Qr)^1.85

P(Q): available pressure at flow Qpsi
Ps: static pressure from the flow testpsi
Pr: residual pressure at the test flowpsi
Qr: the measured test flowgpm

Why the axis is not linear

Friction loss goes roughly as flow to the 1.85 power, so plotting flow on a Q^1.85 axis turns the water supply curve into a straight line. That is the whole point of the convention: a reviewer can see at a glance whether the demand point sits under the supply line, and a designer can read the available pressure at any flow by following the line rather than solving the equation again.

The toggle under the graph switches to a linear flow axis, which is easier to read distances off but bends the supply into a curve. Reports use the Q^1.85 form.

Where a fire pump comes in

If the demand point sits above the supply line, the site needs more pressure than the main will give at that flow. A fire pump raises the curve by its boost at each flow, and the composite of the two is what the demand is then compared against. Pumps are a Pro feature in the full app; this page plots the water supply alone.

Run the whole system, not one pipe. Same solver, pass or fail against your flow test, NFPA 13 report as a PDF.

Try the full calc

Questions about this calculation

What do static, residual and test flow mean?: They are the three readings from a hydrant flow test. Static is the pressure with no flow, test flow is the discharge measured while flowing, and residual is the pressure recorded at that flow. Those three numbers define the whole curve.
Should I apply a safety margin to the test?: Most designers deduct 5 to 10 psi from the static reading to allow for seasonal variation and future demand, and many authorities having jurisdiction require it. The full app has a safety field so the deduction is recorded on the report rather than done in your head.
Does the hydrant elevation matter?: Yes. If the test hydrant is at a different elevation from the base of the riser, the available pressure must be corrected by that difference before it is compared against the demand. This page plots the raw test; the full app asks for both elevations.
How old can a flow test be?: That is set by your authority having jurisdiction, and one year is a common limit. The full app records the test date and location on the report so the reviewer can judge it.
What is the flow at zero pressure?: Where the supply curve crosses the flow axis: the theoretical maximum the main would deliver if the pressure fell to nothing. It is a reference point for reading the curve, not a flow anyone designs to.

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How these results are checked against worked examples · Pricing for the full app

Updated 2026-09-07